Recently, the performance and functionality of digital cameras and digital movie cameras that use some solid-state image sensor such as a CCD and a CMOS (which will be sometimes simply referred to herein as an “image sensor”) have been enhanced to an astonishing degree. In particular, the size of a pixel structure for use in a solid-state image sensor has been further reduced these days thanks to rapid development of semiconductor device processing technologies, thus getting an even greater number of pixels and drivers integrated together in a solid-state image sensor. As a result, the resolution of an image sensor has lately increased significantly from about one million pixels to ten million or more pixels in a matter of few years. Meanwhile, the greater the number of pixels in an image sensor, the lower the intensity of the light falling on a single pixel (which will be referred to herein as a “light intensity”) and the lower the sensitivity of the camera tends to be.
On top of that, in a normal color camera, a subtractive color filter that uses a pigment as a dye is arranged over each photosensing section of an image sensor, and therefore, the optical efficiency achieved is rather low. In a Bayer color filter, which uses a combination of one red (R) pixel, two green (G) pixels and one blue (B) pixel as a fundamental unit, the R filter transmits an R ray but absorbs G and B rays, the G filter transmits a G ray but absorbs R and B rays, and the B filter transmits a B ray but absorbs R and G rays. That is to say, each color filter transmits only one of the three colors of R, G and B and absorbs the other two colors. Consequently, the light ray used by each color filter is only approximately one third of the visible radiation falling on that color filter.
Meanwhile, methods for reading a signal from an image sensor include a non-interlaced method (which is also called a “progressive method” and) in which pixel signals are read on a row-by-row basis and an interlaced method in which pixel signals are read by scanning every other row. According to the non-interlaced method, a Bayer color filter arrangement is generally used. On the other hand, according to the interlaced method, reported are several color representation techniques that can achieve better sensitivity than the Bayer arrangement in a field accumulation mode that mixes together the signals of two vertically adjacent pixels.
For example, Patent Document No. 1 discloses a technique for increasing the sensitivity and resolution using white (W), green, cyan (Cy) and yellow (Ye) color filters. On the other hand, Patent Document No. 2 discloses a technique for reducing false color signals vertically using white, cyan, yellow and other color filters. Furthermore, Patent Document No. 3 discloses a technique for increasing the color reproducibility using red, green, cyan and yellow color filters. And Patent Document No. 4 discloses a technique for realizing good sensitivity and color reproducibility using magenta (Mg), green, cyan and yellow color filters. The technique disclosed in Patent Document No. 4 is currently the mainstream technique in the field accumulation mode.
Hereinafter, the color representation technique disclosed in Patent Document No. 4 will be described with reference to FIG. 10, which illustrates the basic color arrangement of color filters in the image sensor disclosed in Patent Document No. 4. A color filter array is arranged to face a photosensitive cell array, which uses, as a unit block, eight photosensitive cells (pixels) that are arranged in four rows and two columns, and is made up of four kinds of color filters, namely, magenta (Mg), green (G), cyan (Cy) and yellow (Ye) color filters. Pixel signals are read on a two line basis compliant with the NTSC standard for TV signals. In that case, the combination of pixel signals to be read in the second field is shifted by one line from that of pixel signals that have been read in the first field. The pixel signals in two lines are added together only vertically and their sum is processed as a pixel signal representing one line of the first or second field.
In this example, the intensities of photoelectrically converted signals of the light rays that have been transmitted through the magenta, green, cyan and yellow color filters will be identified by Ms, Gs, Cs and Ys, respectively, and their red and blue components will be identified by Rs and Bs, respectively. In that case, since Ms=Rs+Bs, Cs=Gs+Bs, and Ys=Rs+Gs, the signals representing the nth line of the first field will be multiple iterative pairs of the signals Sn,1 and Sn,2 given by the following Equations (1) and (2):Sn,1=Ms+Cs=Rs+Gs+2Bs  (1)Sn,2=Gs+Ys=Rs+2Gs  (2)
On the other hand, the signals representing the (n+1)th line of the first field will be multiple iterative pairs of the signals Sn+1,1 and Sn+1,2 given by the following Equations (3) and (4):Sn+1,1=Ms+Ys=2Rs+Gs+Bs  (3)Sn+1,2=Gs+Cs=2Gs+Bs  (4)
In the second field, these signals are also read in quite the same way. That is to say, the signals representing and n′th and (n+1)′th lines of the second field are also multiple iterative pairs of the signals given by Equations (1) and (2) and multiple iterative pairs of the signals given by Equations (3) and (4), respectively.
A luminance signal YL is generated by adding together signals representing two horizontally adjacent pixels for both of the nth and (n+1)th lines. Also, a color difference signal BY is generated based on the difference between the signals Sn,1 and Sn,2 of the nth line and a color difference signal RY is generated based on the difference between the signals Sn+1,1 and Sn+1,2 of the (n+1)th line. Consequently, YL, BY and RY are represented by the following Equations (5) to (7):YL=Sn,1+Sn,2=Sn+1,1+Sn+1,2=2Rs+3Gs+2Bs  (5)BY=Sn,1−Sn,2=2Bs−Gs  (6)RY=Sn+1,1+Sn+1,2=2Rs−Gs  (7)
As can be seen, according to the color representation technique disclosed in Patent Document No. 4, good color reproducibility can be achieved by performing such signal arithmetic processing. The sensitivity achieved is also better than a situation where a Bayer arrangement is used because the image sensor of Patent Document No. 4 uses color filters of magenta, cyan and yellow that are complementary colors.